1. Field of the Invention
This invention relates to methods of, and apparatus for, eye surgery, and more particularly to a laser-based method and apparatus for corneal and intraocular surgery.
2. Related Art
The concept of correcting refractive errors by changing the curvature of the eye was initially implemented by mechanical methods. These mechanical procedures involve removal of a thin layer of tissue from the cornea by a microkeratome, freezing the tissue at the temperature of liquid nitrogen, and re-shaping the tissue in a specially designed lathe. The thin layer of tissue is then re-attached to the eye by suture. The drawback of these methods is the lack of reproducibility and hence a poor predictability of surgical results.
With the advent of lasers, various methods for the correction of refractive errors and for general eye surgery have been attempted, making use of the coherent radiation properties of lasers and the precision of the laser-tissue interaction. A CO.sub.2 laser was one of the first to be applied in this field. Peyman, et al., in Ophthalmic Surgery, vol. 11, pp. 325-9, 1980, reported laser burns of various intensity, location, and pattern were produced on rabbit corneas. Recently, Horn, et al., in the Journal of Cataract Refractive Surgery, vol. 16, pp. 611-6, 1990, reported that a curvature change in rabbit corneas had been achieved with a Co:MgF.sub.2 laser by applying specific treatment patterns and laser parameters. The ability to produce burns on the cornea by either a CO.sub.2 laser or a CO:MgF.sub.2 laser relies on the absorption in the tissue of the thermal energy emitted by the laser. Histologic studies of the tissue adjacent to burn sites caused by a CO.sub.2 laser reveal extensive damage characterized by a denaturalized zone of 5-10 .mu.m deep and disorganized tissue region extending over 50 .mu.m deep. Such lasers are thus ill-suited to eye surgery.
In U.S. Pat. No. 4,784,135, Blum et al. discloses the use of far-ultraviolet excimer laser radiation of wavelengths less than 200 nm to selectively remove biological materials. The removal process is claimed to be by photoetching without using heat as the etching mechanism. Medical and dental applications for the removal of damaged or unhealthy tissue from bone, removal of skin lesions, and the treatment of decayed teeth are cited. No specific use for eye surgery is suggested, and the indicated etch depth of 150 .mu.m is too great for most eye surgery purposes.
In U.S. Pat. No. 4,718,418, L'Esperance, Jr. discloses the use of a scanning ultraviolet laser to achieve controlled ablative photodecomposition of one or more selected regions of a cornea. According to the disclosure, the laser beam from an excimer laser is reduced in its cross-sectional area, through a combination of optical elements, to a 0.5 mm by 0.5 mm rounded-square beam spot that is scanned over a target by deflectable mirrors. To ablate a corneal tissue surface with such an arrangement, each laser pulse would etch out a square patch of tissue. An etch depth of 14 .mu.m per pulse is taught for the illustrated embodiment. This etch depth would be expected to result in an unacceptable level of eye damage.
Another technique for tissue ablation of the cornea is disclosed in U.S. Pat. No. 4,907,586 to Bille et al. By focusing a laser beam into a small volume of about 25-30 .mu.m in diameter, the peak beam intensity at the laser focal point could reach about 10.sup.12 watts per cm.sup.2. At such a peak power level, tissue molecules are "pulled" apart under the strong electric field of the laser light, which causes dielectric breakdown of the material. The conditions of dielectric breakdown and its applications in ophthalmic surgery had been described in the book "YAG Laser Ophthalmic Microsurgery" by Trokel. Transmissive wavelengths near 1.06 .mu.m and a frequency-doubled laser wavelength near 530 nm are typically used for the described method. Near the threshold of the dielectric breakdown, the laser beam energy absorption characteristics of the tissue changes from highly transparent to strongly absorbent. The reaction is very violent, and the effects are widely variable. The amount of tissue removed is a highly non-linear function of the incident beam power. Hence, the tissue removal rate is difficult to control. Additionally, accidental exposure of the endothelium by the laser beam is a constant concern. This method is not optimal for cornea surface or intraocular ablation.
An important issue that is largely overlooked in all the above-cited references is the fact that the eye is a living organism. Like most other organisms, eye tissue reacts to trauma, whether it is inflicted by a knife or a laser beam. Clinical results have shown that a certain degree of haziness develops in most eyes after laser refractive surgery with the systems taught in the prior art. The principal cause of such haziness is believed to be roughness resulting from cavities, grooves, and ridges formed while laser etching. Additionally, clinical studies have indicated that the extent of the haze also depends in part on the depth of the tissue damage, which is characterized by an outer denatured layer around which is a more extended region of disorganized tissue fibers. Another drawback due to a rough corneal surface is related to the healing process after the surgery: clinical studies have confirmed that the degree of haze developed in the cornea correlates with the roughness at the stromal surface.
The prior art also fails to recognize the benefits of ablating eye tissue with a laser beam having a low energy density. A gentle laser beam, one that is capable of operating at a lower energy density for a surgical procedure, will clearly have the advantage of inflicting less trauma to the underlying tissue. The importance of this point can be illustrated by considering the dynamics of the ablation process on a microscopic scale: the ablation process is basically an explosive event. During ablation, organic materials are broken into their smaller sub-units, which cumulate a large amount of kinetic energy and are ejected away from the laser interaction point at a supersonic velocity. The tissue around the ablated region absorbs the recoil forces from such ejections. The tissue is further damaged by acoustic shock from the expansion of the superheated plasma generated at the laser interaction point. Accordingly, a shallower etch depth or smaller etch volumes involves less ejected mass and acoustic shock, and hence reduces trauma to the eye.
It is therefore desirable to have a method and apparatus for performing eye surgery that overcomes the limitations of the prior art. In particular, it is desirable to provide an improved method of eye surgery which has accurate control of tissue removal, flexibility of ablating tissue at any desired location with predetermined ablation depth or volume, an optically smooth finished surface after the surgery, and a gentle surgical beam for laser ablation action.
The present invention provides such a method and apparatus.